In hyponatraemia, the concentration of sodium in the plasma decreases to below 135 mmol/l, and the condition is considered severe when the concentration falls below 120 mmol/l. The plasma sodium concentration should be checked from all acutely ill patients. It is important to differentiate between acute, clearly symptomatic hyponatraemia that has developed rapidly (< 48 hours) and chronic hyponatraemia that has developed slowly over several days, weeks or even months. The manifestations of acute hyponatraemia are progressive central nervous system symptoms (confusion, seizures, unconsciousness), whereas the symptoms of chronic hyponatraemia can be very slight and more generalised (e.g. lethargy, falls, gait disturbances, impairment of attention). The urgency of treatment is always determined by the patient's symptoms as well as the degree and duration of hyponatraemia. Prevent hypernatraemia by adequate fluid therapy in patients with fluid loss. Avoid too rapid rehydration.Ensure that fluid intake (at least 1.5-2 l/day) is monitored in elderly patients with inadequate thirst control, especially if they have fever and during hot weather.